The invention relates to aluminum alloy products in the form of extruded sections or cold rolled sheets having uniformly grey, light-fast surface and a light reflectivity of at most 50 % as a result of anodizing in an electrolyte. The invention also embraces a process for manufacturing these products.
Various processes are known for achieving a decorative grey color tone on aluminum alloy products; these processes are based on anodic oxidation and do not require additional adsorptive coloring. The resultant color tone is determined by a plurality of process parameters, in particular the composition of the electrolyte, the applied voltage, the type of electric current, density and duration and also the composition of the alloy.
The two stage electrolyte coloring processes represent a many and varied group of processes. In a first stage, an oxide layer about 20 .mu.m thick is produced in a sulphuric acid or sulphuric acid/oxalic acid electrolyte using direct electric current and a current density of 100-200 A/m.sup.2. Following that, in a second stage, using alternating current and a current density of 10-150 A/m.sup.2 in a metal salt solution of suitable composition, metal compounds are precipitated out and deposited such that the said compounds adhere to the base of the pores in the oxide layer thus forming permanent light-fast coloring of the oxide.
A further group of processes for producing light-fast grey tone finish makes use of single stage color anodizing in which direct current at a current density of 70-800 A/m.sup.2 is applied in a special electrolyte to produce oxide layers of natural self-color tone. The color tone obtained depends on the composition of the alloy and on the electrolyte comprising organic acids, if desired with addition of sulphuric acid. The materials used are usually aluminum alloys of the type AlMn, AlMg and AlMgSi.
Using specially selected alloys and observing special procedures in the production of the semi-finished product it is possible to obtain decorative grey tones also with standard anodizing processes. These widely known, cost-attractive anodizing processes employ direct-current at a current density of 80-300 A/m.sup.2 and make use of a sulphuric acid electrolyte, often containing additions of carbonic acid. An aluminum alloy known to date in this connection contains 4.5% silicon and 0.5% magnesium. Using a current density of 150 A/m.sup.2, one obtains with that alloy after 40 minutes of treatment an oxide layer which is about 18 .mu.m thick and exhibits a moderately grey color tone. The light reflectivity, as a measure of the grey tone, amounts to 20%. After an oxidation time of 60 minutes, the oxide layer is 27 .mu.m and exhibits a dark grey, self color finish having a reflectivity of 13%. The reflectivity here was measured in each case using a LANGE UME 1-LFE 1-measuring instrument.
During the production of the semi-finished product, this alloy, however, tends to result in excessive wear on the shaping tools. Furthermore, it is very difficult to maintain close tolerances in color tone and uniformity.